Bangkok governor Pol General Aswin Kwanmuang presided over the opening ceremony at 5pm, inviting visitors to an exhibition on Rattanakosin history at the park's Phraya Yanaprakard building.
He said conservation of Mahakan Fort along with development of its surroundings as a public park was the fruit of six decades of effort, begun in 1959 when City Hall was handed the task.
The project required expropriation of 21 plots spread across nearly 8,000 square metres on which 28 homes were originally built. In 1992, a regulation supporting the expropriation was issued, but by that time the community surrounding the fort had grown in size to 102 homes.
In 1994, home owners began to move out in return for compensation from City Hall. In April this year, after negotiations with the last remaining residents, the final 56 homes were demolished and construction of the park began, Aswin said.
Mahakan was among 14 forts that King Rama I, the founder of Rattanakosin Kingdom and Chakri dynasty, built for protection when establishing Krung Rattanakosin (Bangkok) as the capital of Siam in 1782.
Noting that only two now remain, Mahakan and Phra Sumen Fort, Aswin urged visitors to help keep the newly opened park clean and graceful and alcohol free.
The governor also posted a Facebook invitation to visit.
"The fort area has changed so much from its previous state. Now, it is open, with many green trees, beautiful and safe. Wherever you stand, you can observe the gracefulness of the fort and old city wall," he said in the post.
The park and its buildings will serve as a history centre as well as a public recreational space where cultural activities would be held from time to time, Aswin said. It would become a unique tourist attraction on Koh Rattanakosin, joining Santichaiprakarn Park and Phra Sumen Fort, he added.
The governor also invited members of the public to send any suggestions they had for park improvements to his Line chat app account, @aswinbkk.